1. Field of the Invention
The invention relates to the technical field of amplifier and, more particularly, to an amplification system capable of reducing direct current (DC) offset of a baseband signal in a transceiver.
2. Description of Related Art
A zero-intermediate frequency (IF) receiver is also referred to as a direct conversion receiver to directly convert a radio frequency (RF) signal into a baseband signal. Recently, because of high integrity and low cost, the zero-IF receiver is widely used in a RF transceiver, even in a mobile chip. In the zero-IF receiver, an oscillating signal LO of a local oscillator is mixed with the RF signal because of leakage to an RF port of the receiver. When the receiver and mixer performs a conversion of down frequency, a leaked oscillating signal and the LO are self-mixing to result in an output baseband signal with a large DC offset so as to reduce the performance of the entire receiver.
To overcome this problem, U.S. Pat. No. 6,697,611 discloses a low pass filter (LPF) to obtain the DC offset, and the system subtracts the DC offset obtained from the input signal, thereby reducing the DC offset in the signal. As shown in FIG. 1, the DC offset is obtained by passing the input signal through an LPF 16. In addition, the output of the amplifier 2 is cross coupled to the output of the amplifier 1, and the DC offset is reduced by the first differential output line 24 and the second differential output line 26 since the two amplifiers 1 and 2 have the same gain. The input signal is sent to the amplifier 1 through the first differential lines 20 and the second differential line 22. The amplifier 1 is a differential amplifier. The differential amplifier can have a problem of distortion showing in the FIG. 2. As shown in FIG. 2, when there exists a positive bias voltage between the input differential voltage Vip and the input differential voltage Vin, a bias current of the transistor Tr1 is greater than that of the transistor Tr2. Then, the transconductance of the transistor Tr1 is higher than that of the transistor Tr2 such that an absolute gain between Vip and Von is higher than that between Vin and Vop. Accordingly, the amplifier distortion is presented and easily causes poor linearity.
Therefore, it is desirable to provide an improved method to mitigate and/or obviate the aforementioned problems.